Apparatuses and methods for monitoring stress in steel catenary risers

ABSTRACT

The invention describes apparatuses and methods of monitoring fatigue, structural response, and operational limits in structural components. More particularly, the present invention relates to fatigue, response, and operational monitoring systems on steel catenary risers using optical fiber sensors. The sensors can be pre-installed on new risers, or post-installed sub-sea on existing risers, using a variety of methods.

RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 10/228,385, filed on Aug. 26, 2002. U.S. patent applicationSer. No. 10/228,385 is herein incorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention generally relates to apparatuses and methods ofmonitoring fatigue, structural response, and operational limits instructural components. More particularly, the present invention relatesto fatigue, response, and operational monitoring systems on steelcatenary risers using optical fiber sensors.

BACKGROUND OF THE INVENTION

Oil and gas production in deep and ultra-deep water presents manychallenges, one of them being the design of technical and cost effectiveriser systems (the conduit between the sea floor and the host platform).In many deepwater areas where hydrocarbons are found, such as the Gulfof Mexico, severe current loading is invariably expected. High currentcan generate vortex-induced vibrations (VIV) that give rise to highrates of riser fatigue damage accumulation. As water depth increases,riser designs become more varied and VIV behavior presents one of thebiggest uncertainties facing the riser engineers.

A major concern in offshore oil and gas operations, therefore, isuncertainty as to how much life remains in the riser systems, whether adrilling or production riser. Miscalculations as to remaining life canlead to sudden and catastrophic losses in containment of hydrocarbons.As such, exploration and production companies are more likely to err onthe side of conservatism, for example, choosing to shut-in productionwith million dollar repercussions in revenue, rather than risk failure.

At present, the stress and strains in a steel catenary production riser(SCR) are not monitored, but instead are estimated based on sea currentdata, theoretical models, estimates of boundary conditions, andchangeable structural data. Confidence in the calculations is low and afactor of safety of ten to twenty is applied to the calculated life.Judgment and guesswork are used when predicting whether an existingSCR's production life should be extended. Misjudgment in the remaininglife of a riser could lead to catastrophic loss of containment ofhydrocarbons and the resulting negative impacts would be severe.

Similarly, the fatigue effect of large metocean events on risers is notwell known. Metocean events may include extreme wind speeds or stormsurges from hurricanes and large eddy currents at great depths. Thefatigue of any riser that has experienced these events introduces anadditional level of uncertainty. By monitoring the riser through one ofthese large metocean events, the precise level of fatigue will berecorded and evaluated. This data could also allow better assessment ofprevious fatigue due to large metocean events.

In addition, the soil/pipe interaction of a SCR at the Touch Down Point(TDP), the point where the riser contacts the sub-sea floor, is not wellunderstood. This point is where the greatest changes in stress andstrain exist on the SCR. Strain monitoring at the TDP would improve theunderstanding of this interaction. Once a better understanding isgained, improvements in design and decision-making can be made.

Additionally, operational moves by the platform supporting the SCR cancause significant movement of the TDP adding to suspected trenching andinteraction of the pipe with the sea floor. Large trenches have beenobserved in SCR-pipeline surveys, leading to concerns as to the impacton the serviceability of the riser. Optical strain monitoring wouldsignificantly address this uncertainty and allow for operationalguidance in moving the platform around. Likewise, monitoring of thetop-end of the SCR will assist in guiding operational platformmovements, by prescribing and monitoring acceptable stress anddeformation (inclination) of the top end of the SCR.

Large temperatures encountered in many reservoirs produce temperaturesin excess of 200° F. (93.33° C.) (often as high as 350° F.-176.667° C.)in the riser pipe as the hydrocarbons move up to the surface.Temperatures of this magnitude can cause very large mechanical strainsand cycling of strains as the temperatures fluctuate. This is poorlyunderstood through present theoretical models, and is of great concernin the safe operation of production-type risers and flowlines on the seafloor.

To aggravate conditions, hydrocarbons can often be “sour” in that theyproduce highly corrosive environments on the pipe interior. Whereas somestrategies to counter this include very expensive corrosion resistantalloys (CRA's), monitoring of changes in the wall thickness would be ofgreat importance in safe offshore operations.

Accordingly, there remains a substantial need for a solution to theproblem of monitoring fatigue, operational behavior, and stresses ondrilling and production risers.

SUMMARY OF THE INVENTION

The present invention is directed to apparatuses and methods ofmonitoring fatigue in structural components. More particularly, thepresent invention relates to fatigue monitoring systems on steelcatenary risers using optical fiber sensors. As discussed in more detailbelow, the sensors can be pre-installed on new risers, or post-installedsub-sea on existing risers, using a variety of methods.

In one embodiment, the apparatus comprises a plurality of optical fiberstrain sensors fixed directly to the outer surface of the pipe orconduit to be monitored. In another embodiment, a method for monitoringfatigue on drilling and production risers is described. The methodincludes use of optical-fiber-instrumented clamps and devices that inturn are attached to the pipe or conduit of interest and concern, suchthat the strains in the pipe or conduit can be inferred from themeasurements taken off the attached device or clamp.

In one embodiment, the present invention is directed to an apparatus formonitoring fatigue, structural response, and operational limits on steelcatenary risers comprising:

a multi-strand optical cable that extends down the length of the riser,and

a plurality of optical fiber strain sensors attached to the outersurface of the riser, said sensors being connected to said opticalcable.

Other embodiments of the invention include using a very large and densearray or blanket of strain gauges over a relatively short pipe section,say 1 to 2 feet (0.305-0.610 m) in length, in order to detect smallchanges in wall thickness or pitting caused by erosion or corrosion ofthe inner surfaces.

Yet another application would be to allow for a plurality of gauges onthe riser or extension thereof (the flowlines) on the sea floor (over asimilar spacing and extent as that in a TDP configuration) that could besubjected to significant temperature-induced strain fluctuations. Thelarge mechanical strains caused by temperatures—that could be in therange of 200° F. to 350° F. (93.33° C. to 176.67° C.)—are poorlyunderstood and of concern to safe operation.

Lost or deferred production could run into the millions of dollarsshould the risers, or flow lines, be deemed unserviceable. This hashappened to vertical export riser configurations on major offshoreplatforms in the Gulf of Mexico. The fiber-optical based monitoringconfigurations, and inventions herein could avoid these huge losses inincome, possible threat to the environment from ruptured pipes, and thepossible loss of life.

The foregoing summary has outlined rather broadly the features andtechnical advantages of the present invention so that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter, which form the subject of the invention. It should beappreciated by those skilled in the art that the conception and thespecific embodiments disclosed might be readily used as a basis formodifying or designing other apparatuses and methods for carrying outthe same purposes of the present invention. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth andclaimed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the embodiments of the present invention,and, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 illustrates the movement of the touchdown point for a steelcatenary riser;

FIG. 2 is a diagram showing an embodiment of the system configuration ofthe present invention comprising a computer, an optical black box, and amulti-strand optical cable;

FIG. 3 illustrates an embodiment of an optical fiber sensor arrangementon a steel catenary riser;

FIG. 4 illustrates, in detail, the sensor location and cable break outseen in FIG. 3;

FIG. 5 is a detailed cross section of the multi-strand cable of thepresent invention; FIG. 6 is a cross section of a clamp of the presentinvention;

FIG. 7 shows the result of a comparative test of FBG and electricalstrain gauges;

FIG. 8 illustrates the test setup to assess the performance of thefatigue monitoring system using the piggyback concept in the testsnumbered SCRFT 1-11;

FIG. 9 illustrates the test setup for evaluating the strapping mechanismof the piggyback pipe in the rotation and strap tests numbered SCRFT12-31;

FIG. 10 shows the rotation of the pipe in the series of rotation andclamping tests numbered SCRFT 12-31;

FIG. 11 shows the location of the sensors on the steel model SCR andpiggyback pipe;

FIG. 12 plots the piggyback strains against the distance of the sensorfrom the composite section centroid;

FIG. 13 shows the recorded strains on the piggyback pipe as the pipe isrotated through 90°;

FIG. 14 shows the strains measured from the sensors epoxied directly tothe pipe;

FIG. 15 depicts the location of the sensors in tension tests numberedSCRFT 32-35;

FIG. 16 shows the SCR and piggyback strains during the SCRFT 33 test;

FIG. 17 plots the strains against the distance from the centroidalmoment of area;

FIG. 18 shows the strain values recorded with the increase in clampforce;

FIG. 19 shows the test setup and station locations in the scale modeltest of the SCR system;

FIG. 20 shows the setup at each station in the scale model test;

FIG. 21 depicts the detailed site plan for the scale model test;

FIG. 22 shows the raw strain data recorded during the scale model test;

FIG. 23 shows the filtered data from the scale model test;

FIG. 24 shows the reversals calculated by the rain flow-countingprogram; and

FIG. 25 shows the angle from horizontal for the principle straindirection.

It is to be noted that the drawings illustrate only typical embodimentsof the invention and are therefore not to be considered limiting of itsscope, for the invention will admit to other equally effectiveembodiments.

DETAILED DESCRIPTION OF THE INVENTION

The use of optical fibers to measure strain and compute fatigue onstructural components has many applications. The configuration of thesystem depends on the application, what needs to be monitored andrecorded, and whether the system is pre-installed or post-installed. Thefundamental elements of all the systems are generally the same, with thelocations and arrangement of the sensors being adjusted. Here, anapplication for steel catenary risers (SCR) is described.

In general, the present invention monitors strains on a SCR in the touchdown area using a plurality of optical fiber sensors. A similar array offiber-optics gauges can be used at the top end of a vertical riser orSCR to monitor the behavior, fatigue, and impact of operationalmovements on the risers. The sensors allow the pipe fatigue to bedetermined from measured strain variations, the behavior of the pipe tobe monitored, the touchdown point of the SCR to be located, and theon-bottom behavior (like trenching, and temperature-induced strains) tobe monitored. At present, SCR's are not monitored, their touch-down zoneand Vortex Induced Vibration (VIV) behavior is not well understood, andthey are designed based on theoretical values. This system allows thebehavior of SCR's to be understood allowing informed decisions on theriser to be made, structural failures to be avoided, and massivepossible health, safety, and environmental disasters to be protectedagainst.

The SCR fatigue monitoring system has two installation methods.

When a new SCR is to be monitored, the sensors can be “pre-installed,”that is, the sensors can be fixed to the pipe before installation. Thismethod allows the strain sensors to be epoxied or clamped to the SCR inthe pipe yard or on the deck of the installation vessel. The sensors arethen connected to the main optical cable, as the SCR is being laid, suchas in a J-lay or S-lay operation.

When an existing SCR is to be monitored, the sensors can be“post-installed,” that is, the sensors can be fixed to the pipeunderwater using a remotely operated vehicle (ROV). Several installationmethods can be used. One method allows the sensors to be installedsub-sea on an existing riser using a “piggyback” concept. The piggybackconcept uses clamps, instrumented with strain sensors, which arefastened to the SCR with the underwater ROV. The clamp providessufficient force to act as a composite section with the SCR. While thisinstallation is more complicated, it allows an existing SCR to beinstrumented and monitored.

FIG. 1 illustrates how the vessel heave results in the movement of theSCR touchdown point (TDP), the point where the pipe riser contacts thesub-sea floor. Specifically, FIG. 1 shows a floating production vesssel10 being subject to heave (the rise and fall of the waves or swell ofthe sea) from position A to position B. The heave in the vessel 10 alsocauses the riser 12 to heave or change position, thus moving thelocation of the touchdown point 14 of the riser 12 on the sub-sea floor16. The stress concentrations are shown on the graphical insert,indicated as 18. The maximum change in the stress results in thegreatest amount of fatigue. This occurs at the TDP 14. As such, thepresent invention provides sufficient sensor stations, both before andafter the TDP area, in order to successfully monitor the TDP and changesin stress and strain.

Turning now to FIG. 2, there is shown an embodiment of a systemconfiguration of the present invention. The configuration comprises acomputer 20 and an optical black box 22 located on the vessel, and amulti-strand optical cable 24 that extends down the length of the riser12 to the touchdown area. A plurality of sensors (described below) isconnected to the main carrier cable 24 to record the strains in thetouchdown zone of the riser 12, which are relayed to the computer 20 inreal time. The magnitude and direction of the principle strain and thenumber of stress-strain cycles can be counted and accumulated as totalfatigue. The accumulated fatigue can be compared to known SN curves ofestablished metals to produce a percentage of used fatigued life.

The computer 20 can be off-the-shelf PC's or DAQ-type workstationsdepending on the amount of data interpretation, manipulation or storagerequired.

The optical black box 22 can be purpose built, purchased, or assembledby companies like Astro Technology, a Houston-based specialist infiber-optics technology. It provides the light source, interrogates thesignal to understand the changes in frequency that can be related backto minute changes in the optical fibers (and strain gauges), and maycompensate for known effects on the signals caused by temperatureeffects.

The multi-strand optical cable 24 can be assembled from fiber opticsstrand components and ruggedized and armored by cable companies likeMcArtney in Houston, such that it is protected for the intendedenvironment in practical diameters of ½ inch to ¾ inch (1.27-1.905 cm),and lengths of 10,000 feet (3.048 km) as the particular locationrequires.

In FIG. 3, there is shown an embodiment of an SCR optical fiber sensorarrangement of the present invention. Here, the main carrier cable 24extends from the vessel 10 along the length of the riser 12 to a point40 beyond the touchdown point 14. The optical cable 24 has four breaksout locations 31, 32, 33, and 34, where four separate optical fiberswith sensors “break out” of the cable and run along the SCR 12 at90-degree angles. For example, break out location 31 has optical fibers311, 312, 313, 314; break out location 32 has optical fibers 321, 322,323, 324, and so on for the other break out locations. As discussed inmore detail below, each of the four optical fibers (at each of the breakout locations) will have a plurality of strain sensors.

FIG. 4 illustrates, in greater detail, break out location 33 seen inFIG. 3. The cable 24 is shown running the length of the riser 12. Atcable break out location 33, a connector 50 connects four separateoptical fibers 331, 332, 333, and 334 to the carrier cable 24. One ofthe main functions of the connector is to allow for practical runningand deployment of the system through quick-connect of the pre-installedgauge stations with the main carrier cable while the riser is being madeup. The instrumented pipe sections could be made up ahead of time,whereas the carrier cable could be deployed continuously off a reel onthe deployment platform (J-lay barge; work vessel; drilling rig;production platform; etc.). The connectors can be obtained fromspecialist manufacturers or through consultants like Astro Technology ofHouston. The connectors are about 12 inches in length and 2 to 3 inches(5.08-7.82 cm) in diameter at the maximum thickness location. Theconnector 50 allows each optical fiber to join the main optical cable.The optical fibers 331, 332, 333, and 334 also run along the riser 12 at90-degree angles. Each optical fiber has a plurality of sensors,indicated in FIG. 4 as a diamond. For example, optical fiber 331 hassensors 331-1, 331-2, 331-3, 331-4, 331-5, 331-6, etc., optical fiber332 has sensors 332-1, 332-2, 332-3, 332-4, 332-5, 332-6, etc., and soon for the other optical fibers. The sensors are mounted axially to theriser and at 90 degrees from each other on the circumference of thepipe. The 90-degree interval of strain measurement was derived becausethe FBG sensors, which measure strain at a point, proved to be the mostreliable gauge for the intended dynamic application.

The number of sensors on each optical fiber is a function of thedesigner's desires and his or her need to obtain data points ofinterest. In the present invention, it is preferred to have a total of16 sensors per optical fiber. A significant advantage of the presentconfiguration and the fiber-optics technology is the ability to carrymultiple signals down a single fiber. A practical number of 16 off onestrand was used because it provided the level of redundancy andconfiguration sought in the SCR and related applications. Each sensormeasures the direction of the strain, either circumferentially orlongitudinally, and the magnitude of the strain, both for the pipe intension or in compression. A suitable spacing of the sensors has beenassessed as 10 feet (3.048 m), resulting in a four optical fiber groupcovering 160 feet (48.768 m). Ten feet (3.048 m) was determined by seafloor observations, calculation, and experimentation to be a practicalspacing that is close enough to capture the very sharp changes in stressin termination points in a riser (like the SCR TDP or top-end), and yetbe far enough apart so that multiple locations can be monitored over along-enough expanse of riser to capture a changing termination point, asin the touch down point on the sea floor.

With this spacing, FIG. 4 shows a plurality of station locations 61, 62,63, 64, and so on. (FIG. 4 shows five stations on the SCR, with anadditional station to the left of the pipe.) Each station location willincorporate one sensor from each optical fiber. The data from the foursensors at the same length along the SCR will be combined to give thestrains at that one station location. Specifically, sensor 331-1 fromfiber 331, sensor 332-1 from fiber 332, sensor 333-1 from fiber 333, andsensor 334-1 from fiber 334, are combined at station location 61. Thisarrangement, of course, allows for system redundancy. In addition, thegrouping of the sensors to form a station location could be easilyadjusted to acquire additional data at particular areas of interest.

In this fashion, four separate breakout groups, such as 31, 32, 33, and34, provides coverage for 640 feet (195.072 m) of SCR to be monitored,allowing accurate measurement of the TDP, while having sufficientdistance to either side to allow the TDP to shift in location (with thevessel heave) and still be able to record useful information. As before,640 feet (195.072 m) of coverage around the TDP was determined by seafloor observations, calculation, and experimentation to be a practicallength to capture the very sharp changes in stress in termination pointsin a riser (like the SCR TDP or top-end), and allow for a changing touchdown point on the sea floor.

FIG. 5 shows a cross section of the main multi-strand optical cable 24.The optical cable 24 is comprised of a central strength member 26, whichmay be comprised of metal wire of other materials having suitablestiffness and strength properties. A series 28 a, 28 b, 28 c, 28 d, 28e, 28 f, 28 g, of counter-clockwise, helically wound, armored cables,are arrayed concentrically about the central strength member 26. Asecond group of clockwise helically wound, armored cables 30 a, 30 b, 30c, 30 d, 30 e, 30 f, 30 g, 30 h, 30 i, 30 j, 30 k, 30 l, 30 m arearranged concentrically about the inner set of optical cables 28 a-28 g.The entire assembly is protected by a water-roof, polyurethane jacket32. It will be appreciated that other materials may be used for thejacket. Moreover, it will be appreciated that the entire assembly may befilled with resin or other potting materials to assist in providingstrength and structural integrity.

Each optical fiber will carry the signals from several sensor stationsback to the black box. The cable will break out and be connected to thesensor clamp. Optical fiber sensors have numerous benefits, includingmultiplexing capability, immunity to electromagnetic interference (EMI),and capability to instrument long distances without signal loss. Someother key benefits of fiber-optic sensors over conventional sensorsystems are that they: are lightweight and small in size—about thediameter of a human hair, are rugged and have a long life—the sensorswill last indefinitely, are inert and corrosion resistant, have littleor no impact on the physical structure, can be embedded or bonded to theexterior, have compact electronics and support hardware, have highsensitivity, are multifunctional—they can measure strain, temperature,pressure, and vibration, provide continuous real-time monitoring,require no electric current, and are safe to install and operate aroundexplosives or flammable materials.

As discussed above, the SCR fatigue monitoring system has twoinstallation methods.

When a new SCR is to be monitored, the sensors can be “pre-installed,”that is, the sensors can be fixed to the pipe before installation. Thismethod allows the strain sensors to be epoxied or clamped to the SCR inthe pipe yard or on the deck of the installation vessel. The sensors arethen connected to the main optical cable as the SCR is being laid.Special epoxies have been tried in order to achieve the desired adhesionto the pipe surface in a very aggressive offshore environment overperiods of tens of years. The “clamps” envisioned in this pre-installedconfiguration are different from the post-installed clamp discussedelsewhere. The post-installed clamp has sensors attached to it and theclamp forms the interface with the pipe or conduit of interest.

When an existing SCR is to be monitored, the sensors can be“post-installed,” that is, the sensors can be fixed to the pipeunderwater using a remotely operated vehicle (ROV). Several installationmethods can be used depending upon the SCR configuration.

One method of installing the sensors on an existing riser involves theuse of an instrumented curved plate that is attached to the SCR withsub-sea epoxy. The plates are placed along the length of the riser usingan underwater ROV. The curved plate would be of a compatible material,like corrosion-resistant steel, spaced out at similar distances as the10 foot-like intervals. The length monitored can be less than the 640feet (195.072 m) pre-installed configurations lengths as the uncertaintyin the position of the TDP is removed for post installed systems.

Another method allows the sensors to be installed sub-sea using a“piggyback” concept. The piggyback concept uses clamps, instrumentedwith strain sensors, which are fastened to the existing SCR with theunderwater ROV. The clamp provides sufficient compressive force to actas a composite section with the SCR. With this method, the sensors onthe clamp monitor the strains experienced by the clamps. The strains onthe clamps are recorded, allowing the amplitude and the number ofstress-strains cycles of the SCR to be calculated. The amplitude and thenumber of stress-strains cycles, together with the SN curve of the SCRpipe, allow the fatigue and remaining life of the riser to becalculated. In general, the fatigue assessment tracks the number(“N”-axis in the SN-curve) of stress ranges (“S” axis in the SN-curve)over a period of time to determine the accumulation of damage or“fatigue.” SN-curves are usually experimentally determined fatiguefailure relationships between stress range and cycle numbers. In generalthere are numerous types of SN curves that can be a function of thematerial (type of steel) or detail (like the pipe wall or the weldlocation).

Yet a third method of installing the sensors to the existing risercombines the instrumented clamps and the use of sub-sea epoxy.

Other methods of installing the sensors to the steel catenary risers areknown to those skilled in the art, depending upon the configuration ofthe riser and the protective coating and insulation of the riser.

FIG. 6 shows a cross section of a clamp 70 used in the presentinvention. The clamp 70 consists of upper 72 and lower 74 sectionsfastened together by steel bolts 76. The clamp would be made of acompatible material, like corrosion-resistant metal or inconnel,fabricated by a machine shop familiar with the fabrication of offshoreequipment. Instrumentation of the clamp would be done by specialtyoptical fiber consultants like Astro Technology of Houston, or qualifiedtechnicians within a major operating company like Shell. The forceholding the clamp 70 to the pipe 12 increases as the bolts 76 aretightened. As is known to those skilled in the art, the contact betweenthe pipe 12 and the clamp 70 could be difficult due to surfaceirregularities, ovality in the pipe, and uncertainty in the pipedimensions. Accordingly, the clamp has sufficiently thin wall thicknessto allow flexing to the pipe section of interest. This flexibilityallows the clamp to have good contact even under difficult or irregularconditions. The thread sections for the steel bolts 76 are thicker toallow for the greater stress at these locations. The sensors 331-1,332-1, 333-1, and 334-1 are mounted axially to the clamp 70 and at90-degrees from each other on the circumference of the pipe riser 12.

For ease of ROV installation, the optical fibers should be as flexibleas possible. The four fibers would be epoxied around the side of theclamp, and run together inside a plastic coating to the next clamp,where they would then separate and connect to the sensor location. Forstorage and transportation, the clamps could be stacked side by side andthe plastic-coated fibers looped from one clamp to the next clamp. Oncelowered to the sea floor, the ROV would install one clamp at a time.

As indicated above, another problem of risers is vortex-inducedvibration (VIV). One way to reduce VIV is to increase the inherentdamping of the riser. Increasing damping of a metal riser is difficultto achieve because of its relative high stiffness/rigidity. Inclusion ofcompliant bushings at the interface between joints of pipe is onepossible technique, but is inappropriate for welded risers. In the past,a common way to reduce VIV has been to disrupt fluid flow around thelong slender riser by including helical strakes, fairings, or variousshroud arrangements about the riser. Such devices are called vortexsuppression devices, and are usually installed in the top 500 to 1000feet (152.4-304.8 m) of the riser.

The present invention contemplates use of the fatigue monitoring systemwith such vortex suppression devices, where perhaps a channel or groovefor the cabling is provided under the helical strakes or under thefairings. The two systems can be combined, ensuring VIV suppressionoptimal performance and the security of the optical fiber. For example,one of the helical straights can have a section cut out, allowing the ¼inch (0.635 cm) optical cable to be pushed into the cutout section. Theoptical cable will then vertically route up the riser, being held inplace while allowing VIV suppression.

The above-described invention will be more specifically exemplified bythe following examples that are introduced to illustrate further thenovelty and utility of the present invention but not with the intentionof unduly limiting the same. More specifically, the following examplesdescribe some of the research, testing, and evaluation efforts made indeveloping the optical fiber fatigue monitoring system of the presentinvention.

EXAMPLES

A. Testing of Fiber Optic Sensors

Two types of optical strain sensors were considered for the fatiguemonitoring system of the present invention: Optical Time DomainReflectometer (OTDR) sensors and Fiber Bragg Grating (FBG) sensors. Bothsensors are “within” optical fibers and interrogate returned light togive a strain value. OTDR's record strain along the entire length of theoptical fiber. FBG's record strains over a particular length, typically½ inch (1.27 cm), requiring numerous sensors to cover the riser.

1. Optical Time Domain Reflectometer (OTDR) Sensor

An OTDR would allow continuous strain measurement along the length ofthe riser, avoiding critical fatigue points being missed. Eachconnection and section of pipe would have a stress-strain history,allowing an assessment of fatigue.

The OTDR analyzes back-scattered light. As light passes through thefiber, some light is lost by passing outside the fiber or by beingreflected in the opposite direction to the movement of light. Thisbackward reflection of light within an optical fiber is calledbackscatter. As the optical fiber undergoes a strain, a greaterproportion of the light is back scattered. This backscatter is measuredand converted to a strain.

Tests were conducted to assess the OTDR's suitability for the fatiguemonitoring system. Approximately 3000 feet (0.914 km) of single modeoptical fiber in a Kevlar-reinforced PVC jacket was laid in alarge-radius curve at Ellington Field in Pasadena, Tex. An OTDR thatinterrogates the optical fiber with light having wave lengths on theorder of 1300-1500 nano-meters was used. The cable was bent intosuccessively smaller radius curves, from a radius of 50 feet (15.24 m)to a final radius of 0.5 inch (1.27 cm). The reduced radius resulted inan increased strain on the optical fiber. While the cable was bent, theOTDR strain monitoring equipment was observed.

The OTDR failed to record strains in the fiber while the test radius wasreduced. When the fiber was bent close to breaking point, a reflectedsignal was recorded. Beyond the tight bend, no readings could be taken.When the tight bend was released, the signal measured by the OTDRreturned to zero.

It is concluded that with the present OTDR technology and optical fiber,the sensor is not sufficiently sensitive to record the strains requiredby the fatigue monitoring system. An optical fiber that allowed greaterbackscatter could provide sufficient strain data for the fatiguemonitoring system. A greater backscatter optical fiber was notcommercially available at the time of testing, although it is believedthat this technology will be developed in the next few years.

2. Fiber Bragg Grating (FBG) Sensor

Fiber Bragg Grating (FBG) sensors record strains at specific points inthe optical fiber. Small grooves are cut on the surface of the fiberthat make a sensor that is about ½ inch in length. When a strain isapplied to the sensor, the frequency of light passing through the sensoris shifted. The shift in frequency is proportional to the appliedstrain, the light can be interrogated, and the strain on the sensorcalculated. Each sensor is sensitive to a particular frequency band.Multiplexing assigns sensors different frequencies allowing severalsensors to be placed on each fiber. Using multiplexing and multipleoptical fibers, hundreds of sensors can be used in each system to recordnear continuous strain measurement along the riser.

To test the FBG sensor it was adhered to a pipe. A load was applied tothe pipe and the pipe and sensor underwent a strain. While the pipe wasloaded and unloaded the strain values were recorded.

Previously a comparative test of FBG and electrical strain gauges wasundertaken. The results proved the FBG was at least as accurate andprecise as the electrical strain gauge. The test is presented in FIG. 7.

The strains recorded by the FBG sensor had a strong correlation to thesingle point bending moment calculations. As rapid loads were applied tothe pipe, the strain-measuring device reacted quickly and recorded thestrain correctly. In the comparative test with the electrical straingauge, the FBG proved it was a reliable sensor.

It is concluded that the FBG sensors have the accuracy, precision, andresponsiveness required by the fatigue monitoring system. Usingmultiplexing and multiple optical fibers, sufficient sensors can beplaced on the pipe to give accurate measurements of riser strains. Thesensors recorded the data real time allowing stress-strain cycles to berecorded. The sensors are suitable for use in the fatigue monitoringsystem.

B. Testing of Fatigue Monitoring Concepts

To prove concepts and assess the performance of the fatigue monitoringsystem three series of tests was conducted. Each series was conducted ata particular stage in the development of the system, subsequent seriesof tests building on the results of the previous. Within each seriesseveral sets of tests were conducted to evaluate different concepts.

1. First Series of Testing

The first series of tests was subdivided into three sets of tests. Thefirst set, tested the “piggyback” concept. The piggyback concept uses apipe instrumented with strain sensors, which is strapped to an existingsteel catenary riser (SCR). The straps provide sufficient force for theSCR and the piggyback pipe to act as a composite section. Strains arerecorded on the piggyback pipe allowing the amplitude and the number ofstress-strains cycles of the SCR to be calculated. This providessufficient data to calculate the fatigue of the SCR. The second andthird tests assessed the suitability of ABS plastic and Fiberglass tomodel SCR's in subsequent tests.

The piggyback concept test used a steel box section piggyback pipestrapped to a 20 ft (6.096 m) steel model SCR. The test setup is shownin FIG. 8. The piggyback pipe and the model SCR were each instrumentedwith two strain sensors. A two point vertical load was applied to themodel SCR, resulting in a deflection and strains on the pipes. Strainson the model SCR and piggyback pipe were recorded. Eleven tests wereconducted and are labeled SCRFT 1-11. The loading arrangement wasadjusted and the SCR and piggyback pipe were rotated to provide data atdifferent angles. The concept was tested by predicting the strains onthe SCR from strains recorded on the piggyback pipe.

For the piggyback concept test, the FBG's were adhered to a steel modelSCR pipe. For planned larger scale tests, steel would require anexcessive force to cause the required strains, and a more workablematerial would significantly simplify the process. It was decided totest ABS plastic to assess its suitability to be used in the modeling ofan SCR and piggyback pipe. ABS plastic offered several advantages oversteel—it is easier to handle and work, requires lower loads for a givenstrain, and would allow a larger diameter pipe for the large-scaletests.

The principle concern in using ABS plastic was that the material wouldeither behave plastically or undergo creep. Steel does not exhibiteither of these behaviors. By comparing the maximum bending stress withthe yield strength of the ABS, it was found that the material should notexperience plastic behavior. Creep occurs in all plastics to some extentand is the change in length under constant load. To assess the amount ofcreep ABS pipe would experience, an FBG sensor was epoxied to a 1 inch(2.54 cm) OD ABS pipe. Several loads were applied to the center of thepipe, resulting in a displacement and strain. One hour static loads andloading/unloading cycles were applied to the pipe.

A small amount of creep was experienced for the highest static load overthe longest time period. No creep was observed for the loading/unloadingcycles.

It is concluded that creep could occur in ABS pipe if subjected to largestatic strains over prolonged periods. The proposed test procedure wasamended to reduce the duration of static loading periods. It wasconcluded that for the proposed larger scale test the ABS plastic pipewould be suitable. The dynamic loading of the ABS pipe produced noevidence of creep.

Fiberglass was tested to see if it was a suitable material for modelingof an SCR and piggyback pipe. Fiberglass offered several advantages oversteel in that it is easier to handle and work, requires lower loads fora given strain, and would allow a larger diameter pipe for the largerscale tests. Fiberglass was thought an alternative to ABS should ABSprove unsuitable. By centrally loading a fiberglass pipe, strains wereinduced. The pipe was statically loaded for an hour and loaded/unloadedin 1-minute cycles. No creep was observed.

The piggyback concept test provided sufficient data to compare thepiggyback strains to the model SCR strains. The strains from thepiggyback pipe under predicted the strains on the model SCR pipe.Movement between the piggyback pipe and the model SCR resulted in thepipes not acting as a composite section. The force resisting independentmovement of the two pipes was insufficient. The force of the strapsholding the piggyback pipe to the model SCR should be increased. Furthertesting should be undertaken to assess the most appropriate method forsecuring the piggyback pipe.

It was concluded that ABS plastic would be suitable for the large-scaleSCR test as it would behave elastically under the load conditions, be aneasily workable material, allow the pipe to be 2½-inch (6.35 cm) OD, andwould not experience creep due to the dynamic loading of the test.Fiberglass offered no advantages over the ABS plastic.

2. Second Series of Testing

The second series of tests evaluated the strapping mechanism of thepiggyback pipe. Several tests were conducted with the strappingmechanism and the pipe orientation being adjusted. A separate set oftests was conducted with the piggyback sensor below the SCR, to examinethe effect of the piggyback pipe in tension.

The rotation and strap tests were a continuation of the piggybackconcept tests conducted in the first series, and used the same 20 foot(6.096 m) steep pipe and box section steel piggyback pipe. Two strainsensors were located on the model SCR pipe and two sensors on thepiggyback pipe. A two point vertical load was applied to the model SCR,resulting in a deflection and strains on the pipe. The test setup isshown in FIG. 9. The strapping arrangement was adjusted to:

Determine the most effective method of attaching the piggyback boxsection to the model SCR pipe.

Test the calculation of strains in the SCR based on the piggyback pipedata.

The pipe and piggyback box section were rotated to test:

The systems ability to determine principle strain direction.

The systems ability to determine magnitude of principal strain.

The effect of the piggyback pipe on the centroidal moment of area, andwhether the two pipes would act as a composite section.

The rotation of the pipe is shown in FIG. 10. The angle of rotation wasmeasured from the vertical position. The series of rotation and clampingtests were numbered SCRFT 12 to SCRFT 31.

FIG. 11 shows the location of the sensors on the 20-foot steel model SCRand piggyback pipe. The distance from centroid and the strain induced isalso shown. When a composite section is bent, the distance from thecentroid to an element is proportional to the strain in that element.The centroid will undergo no strain while the point furthest from thecentroid in the direction of bending will undergo the greatest. The red,brown, blue, and green dots are FBG strain sensors. The black dot is thecentroidal moment of area of the Model SCR pipe; the orange dot is thecentroidal moment of area of the composite Model SCR and piggyback pipe.The dashed lines relate the sensor location to the strain induced. Theaddition of the piggyback pipe results in the centroid moving up andstiffening the pipe section, and results in the strain diagram shiftingfrom black to orange.

The green and blue sensors are located directly on the steel pipe. Thered and brown sensors are located on the piggyback pipe. Because thepiggyback sensors are further from the centroid, the strains should begreater.

FIG. 12 plots the piggyback strains against the distance of the sensorfrom the composite section centroid. The theoretical values are inblack, the straight line indicating that the strain is proportional tothe distance from the centroid. If the test values matched thetheoretical values, the strains of the piggyback pipe could be used todetermine the strain on the SCR. The types of straps used are shown incolor. It can be seen that the strapping type affected the strain valuesrecorded. The most successful strap arrangement is shown in brown andmeasured about 90% of the theoretical values. It was concluded that thepiggyback and model SCR pipes were still not acting as a composite pipealthough the addition of extra straps improved the value of strainrecorded.

FIG. 13 shows the recorded strains on the piggyback pipe as the pipe isrotated through 90°. The theoretical values are shown in black and arecalculated as if the two pipes act as a composite section. At anglesclose to the horizontal (piggy 60 and piggy 90) the strains are close tothe theoretical. The strains are relatively low and the force that thestraps are resisting is less. It is concluded that the straps can resistthe force to move the pipes independently and allow the piggybackstrains to accurately predict the model SCR strains.

For angles closer to the vertical position, the strains and the forcesacting to move the two pipes independently are greater. The restrainingforce provided by the straps is exceeded and the pipes do not act as acomposite section.

At 0° to 30° (blue and red dots) the recorded strains are similar.Theoretically, the 0° (blue) strains should be greater as the sensor islocated further from the centroid. It is concluded that the independentmovement force exceeded the maximum restraining force at 30° fromvertical (red dots). Additional strain applied to the model SCR will notbe transferred to the piggyback pipe. In this test, the force resistingindependent movement was insufficient to provide a reliable compositesection at all rotation angles.

FIG. 14 shows the strains measured from the sensors epoxied directly tothe pipe. It can be seen that the strain values recorded for 60, 90, and120° correlate well with the theoretical values. This indicates that thepipe is behaving as a composite section.

The sensors at 0 and 30° from vertical (blue and red) produce slightlyhigher strains than the composite sections theoretical values. Thevalues are greater because the two pipes are not acting as a compositesection. The distance from the 0 and 30° sensors is slightly greater tothe SCR centroid that the composite section centroid. When the SCR isacting independently, the recorded strains show a corresponding smallincrease.

To test the piggyback system under tensional conditions, the model SCRpipe and the piggyback pipe were rotated so that the piggyback pipe wasvertically below the model SCR. A two-point load was applied to the pipeand the model SCR deflected. The tension tests were numbered SCRFT 32 toSCRFT 35. The sensors were located as shown in FIG. 15.

FIG. 16 shows the SCR and Piggyback strains during the SCRFT 33 test.When the load is applied/removed, the piggyback and SCR sensors respondtogether. Strong agreement for the loading/unloading cycles allows thefatigue cycles to be easily recorded. Similar results were experiencedfor the clamp rotation tests, but with compressive negative strains.

The strain amplitude is required to successfully calculate the fatiguein the SCR. In FIG. 17, the strains are plotted against the distancefrom the centroidal moment of area. The piggyback strains (red andbrown) are less than the composite section theoretical values. Using thepiggyback strains to predict the SCR strains would underestimate theamplitude of the fatigue cycles. These results are similar to theresults obtained when the piggyback pipe was in compression.

It is concluded that the force resisting the independent movement of thepiggyback and model SCR pipes is insufficient to result in a compositesection. When the piggyback pipe is rotated to angles closer to thehorizontal, the strains decrease and the strap provides sufficient forcefor the pipes to act as a composite section. The piggyback concept isproved, although a greater strapping force is required to maintain acomposite section at all angles of rotation.

3. Third Series of Testing

Following the completion of the second series of tests, additional workon the forces resisting independent movement of the piggyback pipe wasperformed. Finite element analysis of the tests evaluated thetheoretical limit where the resisting force should be sufficient.

In addition, three new concepts were developed for the piggybacksensors;

A clamp would replace the piggyback pipe. The sensors would be locatedon the clamp; the clamp would be secured to the SCR by a screw thread.The screw thread would allow a greater clamping force to be applied tothe pipe. The clamp has the advantage of acting on both tensional andcompressional parts of the pipe.

The piggyback pipe or clamp could be adhered to the SCR by a sub-seaepoxy. The sensors epoxied to the model SCR provided very reliable andaccurate date, the epoxy would ensure a composite section.

The piggyback pipe could be offset from the SCR. If 100% of thetheoretical strains are not obtainable on the piggyback pipe withstraps, the force could be reduced to zero and the strains calculatedfrom the new known conditions.

Finite element calculation concluded that the strap design could notprovide sufficient force to prevent the piggyback pipe from sliding. Thepiggyback strains are significantly less than the theoretical values,even with a small movement of the piggyback pipe.

FIG. 18 shows the strain values recorded with the increase in clampforce. A clamp force of about 460 lbs. (208.65 kg) results in acomposite section. The piggyback strains increase from zero at no clampstrains to their theoretical values when a composite section isachieved. The second series rotation and strap test results indicatethat a clamp force of about 160 lbs. (72.57 kg) was achieved. Thecalculations agree with the tests results and define the required forceto achieve a composite section.

It was concluded that using additional straps might not guarantee thatthe piggyback pipe will act as a composite section. Additionalcalculations were conducted on a solid clamp, and concluded that itwould be strong enough to hold the SCR without movement. It was decidedto construct a solid clamp for use in the large-scale tests.

Two tests were conducted; the first examined the epoxied clamp or plate,the second examined the offset piggyback pipe. The epoxied clamp wasmodel using ABS plastic. A section of ABS pipe was epoxied to an ABSmodel SCR. Loads were applied to the pipe as in previous tests and thestrains recorded. The strains on the epoxied clamp allowed the strainsin the SCR to be accurately predicted. This provided additional evidencesupporting the piggyback concept.

The concept of an offset piggyback pipe allows the piggyback pipe tobend to the same radius of curvature as the SCR. The strain valuesrecorded on the offset piggyback pipe could be used to calculate thestrains on the SCR. The sensor device would attach to the SCR by twoclamps, one at either end of the offset piggyback pipe. The offsetpiggyback pipe would not come into contact with the SCR.

The sensor on the offset piggyback pipe recorded a strain greater thanthe SCR. As the SCR was deflected, the clamps moved inward, resulting ina greater compressional strain on the offset pipe. The test gave resultssimilar to those anticipated. Detail analysis of these results was notcompleted as it was decided to progress with the clamp and epoxiedmethods.

For the piggyback concept to work successfully, the piggyback pipe andthe SCR need to be a composite section. The strap or clamp force needsto be sufficient to prevent independent movement of the two pipes. Intests SCRFT 12-31, the strap force was insufficient to resist movementat the higher strains. In calculations, a clamp would provide greaterresistance and allow the piggyback concept to work.

The piggyback concept works equally well in compression and tension. Acurved plate epoxied to the SCR would provide sufficient data to allowfatigue monitoring of the SCR. In tests SCRFT 32-35, the restrainingforces resisting independent movement of the two pipes was insufficientto prevent movement.

C. Scale Model Testing

On completion of the concept testing, a scale model test of the SCRsystem was performed. An ABS pipe was suspended from the 30 ft. (9.14 m)high wave tank tower and hung in a catenary shape. FBG sensors wereattached to the ABS pipe. To simulate vessel heave, a computerized ramdynamically agitated the pipe. The strains in the touchdown zone (TDZ)were recorded. The test was conducted to assess the system's ability tocount stress-strain cycles, to evaluate how well the system identifiedthe touchdown point (TDP), and to evaluate the accuracy of strainmeasurement.

The scale model test objectives were to:

Model the SCR to test the fatigue monitoring system before deployment,

Evaluate the system's ability to locate and monitor the TDP,

Evaluate the system's ability to count stress-strain cycles,

Evaluate the clamping system's accuracy in measurement of recordedstrain,

Allow the distance between the sensors for the field system to beassessed,

Test the computer program for operational errors, presentation ofresults, and recording of data,

To acquire realistic test data,

Allow dynamic strains to be measured, and

Use an increased number of sensors to test the data acquisition system.

The test setup is as follows. A 90-foot length of ABS pipe was hung fromthe 30 foot (9.14 m) high wave tank tower. The pipe formed a catenaryshape under its own weight. The TDP and pipe end were approximately25-feet (7.62 m) and 60-feet (18.28 m) from the tower. The test setupand station locations are shown in FIG. 19, the locations at eachstation are shown in FIG. 20, and the detailed site plan is shown inFIG. 21.

Sensor Location and Installation

The strain sensors were adhered to the ABS model SCR pipe at 5 stationsalong the pipe, shown on FIG. 19. A station was defined as a locationalong the pipe where the strains would be monitored. Several sensorswould be placed on the circumference of the pipe at each stationlocation. For initial tests, two sensors at 90° were placed at eachstation location. Additional third and fourth sensors at each stationwere later added. The additional sensors allowed the SCR straincomponents to be differentiated. Tension, bending moment and principlestrain direction can be resolved with four circumferential sensorlocations.

The Model SCR pipe was suspended in the test position. The range ofmotion of the TDP was marked. It was decided to locate five stationsaround the TDP. The locations are given below relative to average TDPposition:

Station Number Location 1 9-ft (2.74 m) in front of TDP (location ofmaximum curvature) 2 3-ft (0.914 m) in front of TDP 3 1-ft (0.305 m) infront of TDP 4 TDP location 5 1-ft (0.305 behind TDP

These locations were chosen to ensure:

The maximum strain changes observed at the TDP will be recorded,

The movement of the TDP will be observed,

The highest static strains will be observed,

At a later date strains due to lateral movement may be obtained.

The sensor locations were verified and the surface abraded. The opticalsensors were fixed to the pipe by an epoxy adhesive. The optical sensorsand the main optical fiber cable were fused once the epoxy hardened.

The SCR pipe was displaced vertically by the computerized hydraulic ram.The ram moved 1 foot (0.305 m) at an angle 10° from vertical. Thisdisplacement simulates the vessel heave and causes the SCR TDP to movealong the pipe. The strains along the pipe changed dynamically and wererecorded by the fiber optical sensors system.

The test setup allows the computerized ram to be rotated about 90°,giving a lateral displacement. The ram will displace the model SCR pipe6 inches (0.153 m) in the horizontal direction. The movement is limitedto 6 inches (0.153 m) in order to minimize the sideways moment on theram. The system's ability to monitor these conditions will be assessed.

The following summarizes the results of the scale model testing. Themethod and results of the strain cycle counting and the strain principledirection and magnitude are explained and assessed.

The model SCR is believed to be a realistic test platform for thesystem. The ABS pipe behaved as anticipated and no material problemswere observed.

The TDP can be found by analyzing the strain data. At the time of thetest, the computer display was not completed. It is planned to displaythe riser showing the TDP in relation to the sensor locations.

The clamp strains allowed the direction of the principle strain to becalculated.

The stress-strain cycles of the clamp matched those of the SCR pipe.

The clamp was unsuccessful at predicting the magnitude of the strains onthe SCR. This resulted from the strain on the pipe being significantlygreater than the ATI tests. The forces to cause a composite section forABS are considerably greater than steel. The clamp did not achieve acomposite section with the ABS pipe.

The 1 to 2 feet (0.305-0.610 m) between the station locations wassufficient to see the movement of the TDP. In scaling up the system forfield deployment, a distance of between 5 to 10 feet (1.524-3.048 m)should be sufficient to observe the movement of the TDP.

The SCR fatigue/strain computer display program was not checked, as itwas not complete at the time of the tests.

The systems recorded dynamic strains well. The strains sensors respondedquickly and accurately to the simulated vessel heave.

The number of sensors installed was significantly increased from theearlier tests. The increased quantity of data provided no additionalproblems for the black box and light interrogation software.

The strain values the computer receives have low amplitude noise. Thisnoise resulted in the rain flow counting software counting additionalhalf cycles at reduced amplitude. To correctly record the stress-strainhalf cycles, a frequency filter was applied to the raw data; thisprevented half cycles from being counted incorrectly.

An important aspect of the fatigue monitoring system is the ability tocount the stress-strain cycles that cause fatigue. The dynamic movementsin the scale model test resulted in dynamic strains in the SCR. Thesedynamic strains allowed the strain cycles to be counted. The proposedmethod of counting the stress-strain cycles looks for reversals in thedata. A reversal is the local maximum of minimum, and results when thetrend changes from increasing values to decreasing values, or decreasingto increasing.

The test data had low amplitude noise. This noise resulted in theprogram counting additional low amplitude half cycles. A frequencyfilter was applied to the raw data preventing erroneous data fromaffecting the count. Filtering the data allowed the correct number ofcycles to be counted.

FIG. 22 shows the raw strain data recorded during the scale model test.The model SCR pipe was raised 1 foot and returned to its originalposition over a ten second period. The displacement was of a constantvelocity and paused at its full range of motion. For final testing themotion was sinusoidal, more accurately modeling vessel heave. Therecorded data set allowed the fatigue cycles and filtering processes tobe tested and refined.

In this test, three sensors were mounted on the clamp at 30°, 60°, and90° from horizontal. The 90° sensor (blue) recorded the highest strainchanges, as it is located furthest from the centroid.

FIG. 23 shows the filtered data. The smaller frequencies of change havebeen removed. This has the effect of removing noise and leaving realdata. Because the motion of the computerized ram paused at the maximumextension, a double peak effect is shown. This effect would not occur insinusoidal or real heave data.

FIG. 24 shows the reversals calculated by the rain flow-countingprogram. A reversal is a point where the trend changes; the time betweenreversals is a half-cycle. It can be seen that the half cycle locationswere successfully located. The number of cycles and magnitude of thecycles can be counted easily. This provides sufficient information tocalculate the fatigue of the SCR.

To calculate fatigue, the principle (maximum) strain is required. Theprinciple strain's direction is required to calculate its magnitude. Foran SCR, the maximum strain will occur at the top and bottom of the pipesection. If the angle between two sensors is known, the principle straindirection and magnitude can be calculated. Where three or more sensorsare used, averaging allows a more accurate value to be recorded.

The calculation of the principle strain magnitude and direction wasassessed in the scale model tests. FIG. 25 shows the angle fromhorizontal for the principle strain direction. Where the strain valuesreturned to zero, the system recorded a peak value, this occurred atevery half cycle. The results show that the principle strain was nearvertical for the “crest of wave” heave but this angle changed in the“trough.” It is believed that the pipe rotated to its “preferred”direction while not loaded and when the load was applied the principlestress moved the pipe to vertical position.

Recording the orientation of the pipe joints will allow the relativerotation of pipes to be assessed. If a riser was instrumented on eitherside of a joint, the directions could be monitored and a change in therelative direction would indicate that the thread of a joint was workingitself loose.

The fatigue monitoring system's determination of the principle straindirection and the counting of stress/strain cycles worked successfully.The measurement of the principle strain magnitude was not successful,but the forces involved are understood. The composite section will beconsiderably easier to achieve with a steel SCR pipe.

The apparatuses and methods described herein allow the loading,behavior, and fatigue of the risers to be monitored and understood,during both normal operations and large metocean events. Failure of theSCR and loss of hydrocarbons could be prevented.

Using optical strain sensors and the relatively compact carrier cables,exploration and production companies can practically measure theinformation needed. Benefits of the present invention include, but arenot limited to, the following:

-   -   Production could be maintained under very severe conditions,        such as the extreme sea currents in the Gulf of Mexico during        1999, because precise and continuous information would be        available to determine riser integrity.    -   Joints could be precisely tracked and optimized by knowing how        much of the fatigue life has been used up—reducing costs in        casing.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions, andalterations could be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

1. An apparatus for monitoring fatigue, structural response, andoperational limits on steel catenary risers comprising: a multi-strandoptical cable that extends down the length of the riser; and a pluralityof optical fiber strain sensors spaced continuously andcircumferentially about a cross-section of the riser and attached to theouter surface of the riser, said sensors being connected to said opticalcable.
 2. The apparatus of claim 1, wherein each of said sensorsmeasures the direction of the strain, both circumferentially andlongitudinally, and the magnitude of the strain, both for the riser intension and in compression.
 3. The apparatus of claim 2, furthercomprising: a computer for recording and analyzing the measurementsreceived from each of said plurality of sensors.
 4. The apparatus ofclaim 1 wherein said plurality of sensors are attached to an area of theriser from about 0 feet to about 640 feet from the surface end of theriser.
 5. The apparatus of claim 1 wherein said plurality of sensors areattached to the touch down area of the riser.
 6. The apparatus of claim1 wherein said plurality of sensors are attached to an area of the riserfrom about 0 feet to about 640 feet from the sea floor end of the riser.7. The apparatus of claim 1 wherein said plurality of sensors areattached to the outer surface of the riser before the riser is installedunderwater.
 8. The apparatus of claim 1 wherein said plurality ofsensors are attached to the outer surface of the riser after the riseris installed underwater.
 9. An apparatus for monitoring fatigue,structural response, and operational limits on steel catenary risershaving vortex suppression devices comprising: a multi-strand opticalcable that extends down the length of the riser; and a plurality ofoptical fiber strain sensors spaced continuously and circumferentiallyabout a cross-section of the riser and attached to the outer surface ofthe riser, said sensors being connected to said optical cable.
 10. Theapparatus of claim 9, wherein each of said sensors measures thedirection of the strain, both circumferentially and longitudinally, andthe magnitude of the strain, both for the riser in tension and incompression.
 11. The apparatus of claim 10, further comprising: acomputer for recording and analyzing the measurements received from eachof said plurality of sensors.
 12. A method for monitoring fatigue,structural response, and operational limits on steel catenary riserscomprising: providing a multi-strand optical cable that extends down thelength of the riser; and installing a plurality of optical fiber strainsensors spaced continuously and circumferentially about a cross-sectionof the riser and attached to the outer surface of the riser, saidsensors being connected to said optical cable.
 13. The method of claim12, wherein each of said sensors comprises an instrumented curved plate.14. The method of claim 13, wherein said step of installing furthercomprises: using sub-sea epoxy and an underwater ROV to attach saidinstrumented curved plates along the length of the riser.
 15. The methodof claim 12, wherein each of said sensors comprises an instrumentedclamp.
 16. The method of claim 15, wherein said step of installingfurther comprises: using sub-sea epoxy and an underwater ROV to attachsaid instrumented clamps along the length of the riser.